1. Field of the Invention
This invention relates to a device for discriminating the focal length of an optical system and, more particularly, to a focal length discriminating device suited to the automatic focusing system of video camera using a zoom lens.
2. Description of the Related Art
A wide variety of auto-focus types have been employed in video cameras, etc. Upon finding the phenomenon that the proportion of the high-frequency component in the video signal obtained from the image pickup element or the like varies as a function of the degree of fineness of the picked-up image, a method for controlling the operation of the focus adjusting ring (hereinafter called "helicoid") of the photographic optical system in such a manner as to maximize that high-frequency component, or the so-called "Hill Climb" method, has been proposed as disclosed in great detail in the NHK Technology Research Report 1965, p. 21, Vol. 17, No. 1, serial No. 86 entitled "Automatic Focus Adjustment of Television Camera by the Hill Climb Servo Method" by Ishida et al.
FIG. 1 in block diagram shows the construction of the principal parts of the automatic focusing system based on this "Hill Climb" method. The system includes a photographic optical system or lens 10, a camera circuit 12 receptive of photo-signals from an image pickup element for producing electrical or video signals, and a high pass filter (HPF) 14 for taking the high-frequency component out of the video signal from the camera circuit 12. Responsive to the high-frequency component from the HPF 14, a detector 16 produces an output whose level represents the degree of fineness of an image of an object to be photographed. Its characteristic curve is shown, in FIG. 2, upward convex with the abscissa in the adjusted position of the helicoid. From this, it is stipulated that the focus state in the position A of FIG. 2 is sharpest. A difference-hold circuit 18 samples and holds the output of the detector 16 (hereinafter referred to as "focus voltage") at predetermined times, producing an output signal representing the differentiated variation of the focus voltage with respect to time. So, the output signal of the difference-hold circuit 18, as shown in FIG. 2, becomes zero at the in-focus position A, and takes positive or negative sign on the front or rear side thereof. The characteristic shown in FIG. 2, regardless of whether focusing is effected down from the closest object distance to infinity, or vice versa, remains the same.
Based on the polarity of the output of the difference-hold circuit 18, a motor drive circuit 20 determines the direction in which a motor 22 serving as the drive source for the lens 10 (specifically speaking, its helicoid) rotates. In some systems the speed of rotation of the motor 22 is also adjusted in accordance with the output level of the difference-hold circuit 18. In such a manner, as the motor or helicoid control loop is recycled, the focus voltage at the output of the detector 16 is climbing up the hill, finally reaching its peak. Then, while fluctuating around the maximum to very small extent, it assumes the stationary state.
The application of the automatic focusing device described above to a case that the focal length of the lens 10 is fixed presents no problem. But, when applied to another case in which the lens 10 is a zoom lens, the characteristic curve of the focus voltage (the amplitude of the high-frequency component of the video signal) differs with different zoom positions as shown in FIG. 3. Therefore, a problem arises in that the response speed of focus adjustment varies with variation of the focal length of the lens 10. This is because the field depth and the image angle vary depending on the focal length. As a result, if, as the gain of the automatic focusing loop shown in FIG. 1 (more specifically speaking, the gain of the motor drive circuit 20) is set to an appropriate value to, for example, the telephoto end, the zoom lens 10 is moved to the wide angle end, the response is very slow. Hence, the time necessary to reach the in-focus position becomes very long. To improve this, a technique has been proposed wherein the zoom lens 10 is provided with an encoder for producing zoom information in the form of an electrical signal, and this electrical signal is used to automatically adjust the above-described loop gain, or, more specifically, the gain of the motor drive circuit 20 so that the motor drive speed is maintained constant over the entire zooming range. As the prior art of this sort, mention may be made of, for example, Japanese Laid-Open Patent Application No. Sho 57-58467.
But, this zoom encoder is to convert the mechanical position of the zoom actuator to an electrical signal. Also because it must be used in each item of zoom lens, an increase in price of the zoom lens is called for. Further, because it is in the lens mounting that the zoom encoder is built with high precision accuracy, there is another drawback that the structure becomes complicated, and the size of the lens mounting is increased.